Males and females differ in most phenotypic measures, including the brain. Substantial advances continue to be made in describing the magnitude and variety of sex differences in the brain which occur at the macro, micro and molecular level. This data combined with a broad based knowledge of the temporal profile and hormonal parameters that constrain the developmental establishment of sex differences, now allow for advancement of mechanistic questions. The rat provides an excellent model for elucidating these basic cellular mechanisms. There are sex differences in a multitude of behaviors, which include cognitive, appetitive, social and reproductive behaviors, and the associated brain regions which include the cortex, hippocampus, preoptic area and hypothalamus. Sexually dimorphic copulatory behavior in the adult provides highly quantitative outcome measures for mechanistic developmental studies. Behavioral studies have defined 3 distinct developmental processes that can be operationally defined as 1) masculinization = capacity for expression of male sexual behavior, 2) defeminization = loss of the capacity to express female sexual behavior and 3) feminization = capacity for expression of female sexual behavior. The first 2, masculinization and defeminization, are required for the development of the complete male sexual behavior phenotype. These are both active hormonally-induced processes that are induced consequent to testosterone secretion from the perinatal testis. The third, feminization, is the default processes that occurs in the absence of a hormonal trigger. We will use these historical constructs for exploration into the cellular and molecular mechanisms that establish sex differences in the brain. Specific Aim #1 focuses on the circuitry of masculinization at the systems and cellular level and explores further the role of PGE2. Aim #2 will exploit a novel tool for masculinizing but not defeminizing the brain and use the Agilent Rat Oligo Microarray to examine profiles of gene expression. Aim #3 explores new endpoints of potential significance to the previously unexplored process of feminization based on pilot data generated using a PowerBlot Phosphospecific Array for protein expression. Disorders and diseases of mental health are profoundly influenced by gender. Understanding at the mechanistic level how sex differences are determined provides an entry point into investigating why sex or gender impacts on the eitology of mental illness.